A pandemic can start when three conditions have been met: a new influenza virus subtype emerges; it infects humans, causing serious illness; and it spreads easily and sustainably among humans.
A pandemic can start when three conditions have been met: a new influenza virus subtype emerges; it infects humans, causing serious illness; and it spreads easily and sustainably among humans.
当以下三个条件具备时可出现全球爆发：出现一种新的病毒亚型，能够感染人并导致严重疾病，能够在人群中迅速持久传播。